Lineage relationship was less clear for COVID-19-enriched PGactive clusters defined by PG responsive genes (clusters 2, 6 and 8), with notable increases in PTGER4 and PTGS2 (or COX2), which encode a proposed target in COVID-19 (ref. 29) (Fig. 1k and Extended Data Fig. 4s,t; Online Atlas). Here, PTGER4 is linked to COVID-19.